WO1995019408A1

WO1995019408A1 - Hydrate inhibition

Info

Publication number: WO1995019408A1
Application number: PCT/GB1995/000052
Authority: WO
Inventors: Simon Neil Duncum; Antony Roland Edwards; Christopher George Osborne
Original assignee: Bp Exploration Operating Company Limited
Priority date: 1994-01-13
Filing date: 1995-01-12
Publication date: 1995-07-20
Also published as: GB9400538D0

Abstract

Formation of gas hydrate e.g. crystalline hydrates from natural gas especially with o.5-5 % carbon dioxide is inhibited or retarded by addition to a medium susceptible thereto of an additive (hereinafter called the Additive) which is (i) an aliphatic (N-heterocyclic carbonyl) polymer, hereafter called the Polymer, and especially also at least one of (ii) a cyclic compound with at least one -(C=O)-O- group in a ring, (iii) an amino carbohydrate, (iv) an amino acid or derivative thereof, (v) an amino alcohol or derivative thereof, (vi) a hydroxy ether or derivative thereof, (vii) a hydroxy acid or a derivative thereof, and (viii) a corrosion inhibitor, (ix) a water soluble polymer of a polar ethylenically unsaturated monomer, and (x) a hydrophilic colloid, the Additive being added in an amount effective to inhibit or retard hydrate formation, to a medium susceptible to hydrate formation.

Description

HYDRATE INHIBITION

The present invention relates to hydrate inhibitors and a method for inhibiting the formation of hydrates in particular to a method for inhibiting the formation of hydrates in the petroleum and natural gas industries. Hydrates are formed of two components, water and certain gas molecules, e.g. alkanes of 1-4 carbons, especially methane and ethane, such as those found in natural gas. These 'gas' hydrates will form under certain conditions, i.e. when the water is in the presence of the gas and when the conditions of high pressure and low temperature reach respective threshold values. The gas may be in the free state or dissolved in a liquid state, for example, as a liquid hydrocarbon.

The formation of such hydrates can cause problems in the petroleum oil and natural gas industries. Hydrate formation in the field may cause blocked pipelines, valves and other process equipment.

The problem is particularly of concern as natural gas and gas condensate resources are discovered where operating conditions surpass these threshold values, i.e. in deep cold water and on-shore in colder climates .

Hydrates can also form in association with the underground hydrocarbon reservoir thus impeding production by blockage of reservoir pores.

The problem of hydrate formation is however commonest during gas transportation and processing, the solid hydrate precipitating from moist gas mixtures. This is particularly true with natural gas which when extracted from the well is normally saturated with water. Often in such a case, in a cold climate, hydrates will form in downstream transportation networks and this can cause large pressure drops throughout the system and reduce or stop the flow of natural gas.

Hydrate formation may also occur during natural gas cryogenic liquefaction and separation.

A typical situation where hydrate formation can occur is in offshore operations where produced fluids are transported in a long vertical pipeline, for example, a riser system. Such produced fluids normally include light gases known to form hydrates and water. In such a situation a temperature of 4.5^aC and a pressure of 150 psi would be sufficient for hydrate formation. Several methods are known to prevent hydrate formation and subsequent problems in pipelines, valves and other processing equipment.

Physical methods have been used, e.g. increasing gas temperature in the pipeline, drying the gas before introduction into the pipeline, or lowering the gas pressure in the system. However, these techniques are either expensive or are undesirable because of loss of efficiency and production.

Chemical procedures have also been used. Electrolytes, for example, ammonia, aqueous sodium chloride, brines and aqueous sugar solutions may be added to the system.

Alternatively, the addition of methanol or other polar organic substances, for example, ethylene glycol or other glycols may be used. Methanol injection has been widely used to inhibit hydrate formation. However, it is only effective if a sufficiently high concentration is present since at low concentrations there is the problem of facilitation of hydrate formation. Also for methanol to be used economically under cold environmental conditions there must be early separation and expulsion of free water from the well in order to minimise methanol losses in the water phase. We have now found certain additives which may be used as effective hydrate inhibitors at low concentrations.

Thus, according to the present invention, there is provided a method for inhibiting or retarding hydrate formation, which method comprises adding an additive (hereinafter called the Additive) which is (i) an aliphatic (N-heterocyclic carbonyl) polymer, hereafter called the Polymer, and preferably also at least one of (ii) a cyclic compound with at least one -(C=0)-0- group in a ring, (iii) an amino carbohydrate, (iv) an amino acid or derivative thereof, (v) an amino alcohol or derivative thereof, (vi) a hydroxy ether or derivative thereof (vii) a hydroxy acid or a derivative thereof, (viii) a corrosion inhibitor, especially a quaternary ammonium corrosion inhibitor, (ix) a water soluble polymer of a polar ethylenically unsaturated compound different from Additive (i) and (x) a hydrophilic colloid, the Additive being added in an amount effective to inhibit or retard hydrate formation, to a medium susceptible to hydrate formation. When the medium in a gas comprising 0.5-5% by weight of carbon dioxide Additive (i) may be added thereto.

The Polymer is an aliphatic (N-heterocyclic carbonyl) polymer with a hydrocarbon backbone. It is water soluble or water dispersible, eg to an extent of at least 0.01% by weight in water such as at least 0.05% but especially at least 0.5%, such as up to 10% by weight. Its molecular weight is usually 5000 to 1000000 eg 10000 to 1000000 such as 1000 to 50000 and preferably has a K value of 10 - 150 especially 15 - 50, wherein the K value is obtained from the relative viscosity in aqueous solution via the FIKENTSCHER' S Formula, from which the average molecular weight is calculated as described in USP 2,811,499. The Polymer has a hydrocarbon chain with pendant N-heterocyclic carbonyl groups, with the bonding to the chain via the heteroring -N- atom. The carbonyl group may be in any position in the N heteroring, but is especially alpha to the N hetero atom, so the N-heterocyclic rings are preferably derived from lactams , especially of 4-8 eg 5 or 6 ring atoms, such as those derived from butyric, pentenoic, pentanoic or hexanoic acid lactams (or 2-pyrrolidone , 2-pyridone, 2-piperidone or omega caprolactam) . The aliphatic group or groups in the polymer may be part of the hydrocarbon chain, or bonded to it or to the N-heterocyclic carbonyl ring; the aliphatic group may be linear or branched and maybe alkyl eg of 1 - 40 eg 2 - 25 carbons or alkenyl eg of 2 - 20 carbons, especially methyl, ethyl, butyl or octyl, tetradecyl, hexadecyl, octadecyl, eicosyl, tricosyl or ethylene, butylene or octylene. The molar ratio of aliphatic group to heterocyclic carbonyl group in the Polymer is usually 1:99 to 20:80 eg 5 -15 : 95-85.

The Polymer may be a copolymer having repeat units derived from at least one monomer which is an optionally alkyl substituted vinyl N-heterocyclic carbonyl compound) and at least one monomer which is an olefin; this copolymer may be simple copolymer formed by copolymerization of the monomers or a graft copolymer formed by grafting the olefin onto a polymer of the N-heterocyclic monomer. The Polymer may also be an alkylated derivative of a polymer of an optionally alkyl substituted (vinyl N-heterocyclic compound) especially a homopolymer of such a compound.

The optionally alkyl substituted vinyl N-heterocyclic carbonyl compound may be of general formula:

_R10 _Rll _c . _CR12 _R13 wherein each of R^- , R^ and R-*--³ , which may be the same or different, represents a hydrogen atom or an alkyl group eg of 1 - 20 carbons , such as methyl, ethyl, butyl, hexyl, decyl or hexadecyl, and R^-0 represents an N-heterocyclic carbonyl group with the free valency on the N atom; preferably the N heterocyclic carbonyl group is as described above. The N-heterocyclic ring may contain 1-3 ring N atoms but especially 1 ring N atom and 0 - 2 other ring hetero atoms eg 0 or 5 , but especially no ring hetero atom; the ring may contain in total 1 or 2 rings , which may be saturated or ethylenically unsaturated such as a pyrrolidine, piperidine, quinoline or pyridine ring. Preferably RH , R^-2 and R-^ are hydrogen and R^0 represents an N- (pyrrolidone) , N-(2 pyrid-2-one) or N- (piperid-2-one) group.

The olefin is usually of 2 - 32 eg 4 - 18 carbon atoms and is generally a hydrocarbon. It is preferably an alkene , especially a linear alkene and has in particular a terminal olefin group. It is preferably a vinyl olefin eg of formula CH2 = CH - R^> where R-^ is hydrogen or alkyl of 1 - 40 carbons, such as methyl, ethyl, propyl, butyl, hexyl or decyl , tetra decyl, octadecyl or octacosyl (so the olefin is tricosene) . The olefin is preferably butylene octene-1 or dodecene-1, hexa decene-1, octadecene-1, eicosene-1 or tricosene-1. The Polymer may be made by free radical copolymerizing the N- heterocyclic carbonyl compound eg N-vinyl pyrrolidone with the olefin eg butylene in solution in the presence of a peroxide catalyst. The Polymer may also be made by free radical grafting of the olefin onto a polymer of the N-heterocyclic carbonyl compound eg poly vinyl pyrrolidone (PVP) with K value as described above. The copolymerizations may incorporate structural units from the olefin into the hydrocarbon polymer chain and/or insert such units into the N-heterocyclic rings.

The Polymer may also be made by direct alkylation of the polymer of the N-heterocyclic carbonyl compound eg PVP with an alkylating agent eg an alkyl halide such as butyl bromide or octyl bromide, optionally in the presence of a base, such as triethyl amine.

Finally the Polymer may be a homo or copolymer of an alkyl substituted N-(alkenyl) heterocyclic compound in which the alkyl substitutuent may be in the N-heteroring and/or present in the alkenyl side chain; the alkyl substituent may be as is preferred for the aliphatic group on the Polymer described above. The Polymer may have structural units from an N-vinyl-alkyl ring substituted heterocyclic carbonyl compound, such as N-vinyl-3-methyl pyrrolid-2- one and/or from an N-butenyl-heterocycle carbonyl compound, such as N-butenyl-pyrrolid-2-one. Such Polymers may be made by polymerization in solution in the presence of a free radical catalyst, in an analoguous way to polyvinyl pyrrolidine. Thus the preferred Polymers are aliphatic (N-heterocyclic carbonyl) polymers with units derived from N-vinyl pyrrolid-2-one and butylene (sold as Antaron P 904), octylene, dodecylene, hexadecylene (sold as Antaron V216) , eicosylene and tricosylene; the Antaron products are sold by International Speciality Products of Wayne, N.J. , USA. Additive (ii) is a cyclic compound with at least one -(C-O)-O- group in a ring. Such additives are usually cyclic lactones or carbonate esters, preferably with 4-8 atoms in total in the ring containing the -(C=-0)-0 group. The additives are preferably of formula -R⁶-O_a(C=0) -0- , where a is 0 or 1 and R^ is a divalent organic group especially with at least one chain of carbon atoms, whose terminal members are bonded to the -(0)_a(C-0)-0- group. The group R° is preferably a hydrocarbyl group, e.g. of 2-20 carbons, especially 2-8 carbons. R° may be an alkylene group of 2-10 carbons, preferably forming with the - (0)_a(C-=0)-0- group a 4-8, especially 6 or 7 membered ring in particular in lactones; Group R° is especially an alkylene group with one terminal CH2 group, in particular that attached to the non carbonyl oxygen atom in a lactone, but preferably group R° is an alkylene group with 2 terminal CH2 groups R° may be 1,2 ethylene, 1,3 propylene, 1,4 butylene, 1,4-pentylene, 1,5- pentylene and 6-methyl-l,5-pentylene. R" may also be cycloalkylene group e.g. of 5-7 carbons especially a 1,2-cycloalkylene group, such as cyclohexylene, or an alkylenecycloalkylene group or alkylenecycloalkylene alkylene group, each preferably of 5-10 carbons with 1-3 carbons in the or each alkylene group; examples of such groups are 1,2- cyclohexylene, and 2-methylene,1-cyclohexylene and cyclohexane 1,2-methylene, R° may also be a divalent aromatic group e.g. of 6-10 carbons such as a divalent hydrocarbyl group e.g. 1,2- phenylene or an alkylene aromatic group or alkylene aromatic alkylene group, with the or each alkylene group of 1-3 carbons and the aromatic group as defined above, examples being 1-phenylene 2- methylene and phenyl 1,2-bismethylene. If desired the group R° may be attached to a hydrocarbyl polymer with the or each -0_a(C-0)-0- group present as part of a ring, pendant on or fused to the hydrocarbyl polymer chain. The rings usually have 5-8 ring atoms. The above groups R° may be unsubstituted by any non hydrocarbylsubstituents , and any alkyl chains present are usually of less than 8 e.g. less than 4 carbons.

Examples of the cyclic compounds which are lactones are propiolactone, gamma butyrolactone, gamma valerolactone, 6- caprolactone and the lactone of 2-hydroxymethyl benzoic acid, while examples of the cyclic carbonates are those from ethylene glycol, 1,3-propylene glycol, 1,2-propylene glycol, 1,2 and 1,3-butanediols, cyclohexane 1,2-diol and o-catechol, though polyvinyl carbonate may also be used e.g. that derived from "polyvinyl alcohol" (formed by hydrolysis of polyvinyl acetate) .

Additive (iii) is an amino carbohydrate in which an amino group is preferably a secondary or especially primary amino group (NH2) , especially the additive containing at least 1 amino group per carbohydrate ring. The amino group is usually bonded directly to a carbon atom in the carbohydrate ring or is separated therefrom by a carbon atom e.g. as in an amino methyl carbohydrate. The Additive contains at least one carbohydrate ring which may be a pentose or especially a hexose ring, and the amino group is especially a 2-amino group in the ring. Examples of such amino carbohydrates are glycosamine and fructosamine, as well as polymers thereof such as chitosan, (polyglucosamine, formed by enzymic hydrolysis of chitin) ; the polymers may have molecular weights of 10,000 to 5 million e.g. 30,000-200,000 or 100,000-5 million such as 500,000 to 3 million. The polymeric amino carbohydrate may be used in association with acetic acid eg with 0.5 - 10 times the weight of acetic acid.

The Additive (i) is used alone or preferably mixed with at least one of the additives (ii)-(x), preferably (viii) and/or (x) , and especially also (iii) and/or (v) and/or (vi) . The Additive (iv)-(vii) (and often viii) is usually at least difunctional, e.g. with 2-4 functional groups. It preferably has a structure including an oxygen atom in an ether group or hydroxyl group and at least one of another oxygen atom in an ether or keto group and a nitrogen atom in an amino group, said 0 and N and/or 0 atoms being spaced by 1-6, preferably 2-5, especially 2 or 3 carbon atoms, which may be in an aliphatic, cycloaliphatic or aromatic group.

The Additive (iv) may be an amino acid or derivative thereof, in particular one with at least one asymmetric carbon atom; the Additive may be in the racemic form, but is preferably optically active, i.e. in D or especially -form. The Additive (iv) may be in the form of the free carboxylic acid (including a carboxylic anion form, e.g. as a sodium or potassium salt) or as a derivative of said carboxylic acid, e.g. as an amide (e.g. with ammonia or with a primary amine which may or may not be a hydroxyamine) or hydrazide

(e.g. with hydrazine) or an ester, e.g. as an alkyl ester, e.g. with 1-6 carbons such as a methyl or ethyl ester, or a hydroxyalkyl ester, e.g. with 1-6 carbons such as hydroxymethyl, 1 or 2 hydroxyethyl, 2 or 3 hydroxypropyl ester. The amino acid (or derivative) is preferably an alpha amino carboxylic acid, especially with the amino and carboxylic acid groups bonded to an aliphatic carbon atom, especially a -CH- group. The amino group in the amino acid may also be in a non alpha position relative to the acidic group, e.g. in a beta, gamma or delta position, especially terminal on an alkyl, phenyl or phenylalkyl group attached to the acidic group; examples are omega amino alkanoic acids such as 4-amino butyric acid and p- amino phenyl acetic acid. Iminodiacetic and N-hydroxy methyl iminodiacetic acid may also be used. The Additive may contain an amino group in H2 or NH3 form (whether as a salt, e.g. hydrochloride or hydrobromide or zwitter ion form) or an N-acyl derivative thereof, such as with an alkanoic acid, e.g. of 1-10 carbons such as acetic acid, or an aromatic acid, e.g. of 6-16 carbons such as benzoic acid or a carbonate half ester such as a monoalkyl carbonate, e.g. mono tert butyl carbonate or an aromatic or aralkyl carbonate, e.g. phenyl carbonate or benzyl carbonate. Derivatives of the amino acid with an N-hydroxy alkyl group, e.g. of 1-4 carbons, e.g. 1 or 2 carbons such as hydroxymethyl may also be used; such derivatives may also involve an NH ring in which the hydroxy 1 and acid groups are substituents as in hydroxy proline. Examples of such amino acid derivatives with an N-hydroxy alkyl group are N-sulphoalkyl and N-carboxyalkyl derivatives of Additives (v) as further described below; such compounds may be 1, 2 or 3 sulphoalkylene or especially 1,2 or 3 carboxyalkylene derivatives, especially linear alkylene derivatives. Especially preferred are those derivatives which are N-derivatives of a tris(hydroxyalkyl) alkylamine, in particular tris(hydroxymethyl) methylamine, such as N-[(tris hydroxymethyl)methyl] glycine.

The Additive (iv) is however preferably of formula (I) R- CH(NH2)COOH (or a derivative thereof) , wherein R represents hydrogen or an aromatic, aralkyl, heterocyclic, heterocyclic aliphatic, cycloaliphatic or aliphatic group. The aromatic group preferably has 6-20 carbons, e.g. 6-10 carbons, such as phenyl optionally substituted by amino, hydroxyl carboxylic or alkyl, e.g. of 1-6 carbons such as methyl or ethyl or alkoxy, e.g. of 1-6 carbons such as methoxy or ethoxy. The aralkyl group preferably comprises an aromatic group as described above and the alkyl part of the aralkyl group preferably has 1-4 carbons such as methyl or 1 or 2 ethyl. The heterocyclic ring in the heterocyclic or heterocyclic alkyl group may be a nitrogen heterocyclic ring with 1-3 ring nitrogens as in a 1,3- imidazole ring. The cycloaliphatic group may have 5-8 carbons, e.g. cyclohexyl or cyclopentyl. The group R is especially an aliphatic group, e.g. of 1-6 carbon atoms, in particular a saturated hydrocarbyl group, e.g. linear or branched and optionally substituted, especially in a terminal position in the hydrocarbyl group by at least one of a hydroxyl, alkoxy, mercapto or alkyl thio group, e.g. with 1-6 carbons, especially 1 or 2 carbons in the alkoxy and alkyl thio group, or an amino group, e.g. of formula -NR^-R^. wherein each of R-*- and R^ which may be the same or different represents hydrogen or alkyl of 1-6 carbons such as methyl or ethyl, or may be an NHC = NH- ( H2) group (as such or in a salt form) or a ureido or carboxylic acid group or ester (e.g. alkyl or hydroxyalkyl ester) or hydrazide or amide thereof.

Thus the amino acid which is the Additive (iv) or from which it is derived, may be a basic amino acid with 2 or 3 amino groups and 1 carboxylic acid group, such as ornithine, lysine, arginine or guanidine or histidine, especially in its L-form, or an acidic amino acid with 2 or 3 carboxylic acid groups and 1 amino group, such as glutamic or aspartic acid or amide, especially in its L-form. Most preferably, however, the amino acid, which the Additive (iv) is or from which it is derived, has 1 amino and 1 carboxylic group (or derivative thereof) especially one of formula I, in which R is alkyl or hydroxyalkyl, alkoxyalkyl, mercaptoalkyl or alkylmercaptoalkyl group or in the case of mercaptoalkyl groups (as in cysteine) the corresponding disulphide (as in cystine) ; such amino acids are preferably in their L forms. The Additive is thus preferably serine, threonine, valine, leucine, isoleucine, homoserine, methionine, cysteine or cystine; L-serine is preferred.

The Additive of formula 1 may also be of formula II

XY₂Z₂C₆-(CH₂)_n-CH(R )-C0R³ II

Wherein in the Cg benzene ring in relation to the -(CH2)_n group X is para, both Y are meta and both Z are ortho, and R³ is OH, OCH3, OC2H5, NHNH or H, R⁴ is NH₂ or NH3, each of X, Y and Z is hydrogen or OH and n is a number in the range 0 to 6, or a polymer thereof when R³ is NH2 and may be phenylalanine or tyrosine or their carboxylic acid derivatives. There may be used compounds of formula II, wherein at least one of X, Y and Z represents an alkoxy, acyloxy, keto, amino or carboxyl group such as an alkoxy group of 1-6 carbons, e.g. methoxy or ethoxy, or acyloxy such as aliphatic acyloxy, e.g. of 1-10 carbons such as acetoxy, or keto as in alkylketo with 2-6 carbons, e.g. CH3CO- . Advantageously, however, in the additive of formula I, group R does not comprise an aromatic group.

The amino acid may also be an amino sulphonic acid, e.g. with an aliphatic group, e.g. of 1-10 carbons, aromatic group, e.g. of 6- 16 carbons or cycloaliphatic group, e.g. of 5-7 carbons, to which the amino and sulphonic groups are attached. Taurine and amino benzene sulphonic acid may be used.

The Additive (v) may also be an hydroxyamine, which is usually an aliphatic compound with 2-10 carbon atoms, especially 3-6 carbon atoms, in particular a saturated aliphatic compound. The Additive (v) may contain 1-5 hydroxyl groups, e.g. 1, 2 or 3 hydroxy groups and 1-3 amino groups, e.g. 1 or 2 amino groups, which may be of formula NR^-R^, where R^- and R^ are as defined above. The Additive (iv) may be a secondary or tertiary amine, but is preferably a primary amine, e.g. with 1 NH2 group, and with 1-3 hydroxyl groups and has especially an alkane group, which may be linear or branched, substituted by such amino and hydroxyl groups which are preferably attached to adjacent carbon atoms. The hydroxyamine may be used as such or in the form of a salt with an inorganic acid, e.g. hydrochloric acid or an organic acid, e.g. an aliphatic mono or di carboxylic acid such as one with 1-10 carbon atoms, e.g. acetic or propionic acid or maleic or oxalic acid or an organic sulphonic acid or organic sulphate half ester, each of which usually contains 1-20 carbons in the organic group, which is preferably a hydrocarbyl group such as alkyl e.g. of 1-8 carbons such as methyl or ethyl especially for the sulphonic acid, or of 1-24 such as 8-22 carbons such as octyl, lauryl and stearyl, especially for the half ester, or may be aryl of 6-20 carbons such as phenyl or tolyl or aralkyl of 7-21 carbons such as benzyl. Examples of such Additives (v) are tris- (2 hydroxyethyl) amine, tyrosinol or valinol (e.g. the L isomer) and especially tris-hydroxymethyl-methylamine (H0CH2)3CNH2 and its corresponding ammonium chloride and lauryl sulphate salt.

The Additive (vi) may also be an ether, which contains at least 1 such as 1-6 ether oxygen atoms and 0-6, e.g. 1 oxygen atoms in a hydroxyl group, the number of ether oxygen atoms being preferably at least the same as and especially 0-3 more than the number of hydroxyl oxygen atoms. In the ether, the ether 0 atom is usually bonded to 2 carbon atoms , each of which is bonded to another carbon or hydrogen atom only. Preferably the Additives (vi) are glycol ethers, derived structurally from a diol, e.g. an aliphatic diol of 2-4 carbon atoms such as ethylene glycol or 1, 3-propylene glycol, or structurally from a self condensate thereof, e.g. di or triethylene glycol, or mixtures of said diols . One or both hydroxyl groups from such diols (or condensate thereof) may be etherified, e.g. with an alkyl group such as one with 1-10 carbons, e.g. methyl, ethyl, n-propyl or n-butyl. Preferably such Additives (vi) are the reaction products of an alkanol with ethylene oxide and/or propylene oxide. One or more hydroxyl groups in the ether may be acylated, e.g. with an aliphatic or aromatic acyl group with 1-10 or 6-16 carbon atoms respectively such as acetyl or benzoyl. Examples are 2-butoxyethanol (monobutyl glycol ether) and 2(2^--butoxy ethoxy) ethanol (mono butyldiethylene glycol ether) and their corresponding acetates.

The Additive (vii) is a hydroxy acid (especially a hydroxy carboxylic acid) which may be an aliphatic, aromatic, cycloaliphatic, cycloaliphatic-aliphatic or araliphatic compound with 1-20, 6-20, 5- 7, 5-25 or 7-20 carbon atoms respectively. Preferably the aliphatic hydroxy acid is an alpha hydroxy acid, e.g. of 2-6 carbons such as glycollic, lactic or citric acid, while the aromatic and araliphatic acids may be salicylic, p-hydroxybenzoic or 2-(p-hydroxyphenyl) propionic acid. Derivatives of the additive (vii) on the carboxyl group may be used such as salts, e.g. with alkali metals, esters[e.g. with an alcohol with 1 hydroxyl group or with a polyol (though esters from polyols are preferably absent) ] , amides or hydrazides, in particular when the additive (vii) contains at least one group -COX where X is as defined below. However, preferably the hydroxy acid is a cycloaliphatic-aliphatic compound, especially one with at least one of a keto and an olefinic unsaturated group, in particular abscisic acid or a derivative thereof of formula III.

l-hydroxy-2,2dimethyl-4keto-cyclohex-5-en-l-yl-CH-CH-C(CH3)-CH-C(0-0)X¹ (II

wherein χl represents OR-*, wherein R^ is hydrogen or alkyl, e.g. of 1-6 carbons optionally hydroxy substituted, such as methyl, ethyl or 2-hydroxyethyl, or X-*- represents amino (e.g. NH2) or hydrazido (e.g.NHNH₂).

All isomers of the aforementioned abscisic acid derivatives may be suitable for use in the method of the present invention. A preferred compound of formula III is (+/-) -2-cis,4-trans abscisic acid. The additive (viii) is a corrosion inhibitor. It is preferably a quaternary ammonium salt though, when at least one Additive (ii)- (vi) , (ix) and (x) is present, it may also be any other corrosion inhibitor. They are corrosion inhibitors e.g. for steel and are usually ones suitable for use in anaerobic environments, and especially nitrogenous ones with 1 or 2 nitrogen atoms. The corrosion inhibitor may be a primary, secondary or tertiary amine, or a quaternary ammonium salt, usually in all cases with at least one hydrophobic group, usually a benzene ring or a long chain alkyl group e.g. of 8-24 carbons; the inhibitor preferably has surfactant activity and especially surface wetting activity. It may be a quaternary ammonium salt, a long chain aliphatic hydrocarbyl N- heterocyclic compound or a long chain amine. The quaternary salt may be an (optionally alkyl substituted) benzyl trialkyl ammonium halide, in particular when at least 1 and especially 1 or 2 alkyl groups is of 1-20, in particular 8-20 carbons such as cetyl and the other alkyl groups are of 1-6 carbons such as methyl or ethyl; examples are benzyl alkyldimethyl ammonium chloride and Benzalkonium chlorides (e.g. mixtures of benzylalkyldimethyl ammonium chlorides, especially with alkyls of 8-18 carbons). The aliphatic hydrocarbyl group in the heterocyclic compound usually has 8-24 carbons in the hydrocarbyl group, preferably a linear saturated or mono or diethylenically unsaturated hydrocarbyl group; cetyl-, stearyl and especially oleyl- groups are preferred. The N-heterocyclic compound usually has 1-3 ring N atoms, especially 1 or 2 which usually has 5-7 ring atoms in each of 1 or 2 rings; imidazole and imidazoline rings are preferred. The heterocyclic compound may have the aliphatic hydrocarbyl group on an N or preferably C atom in the ring; the ring may also have an amino-alkyl (e.g. 2-amino ethyl) or hydroxyalkyl (e.g. 2- hydroxyethyl) substituent, especially on an N atom. N-2-aminoethyl- 2-oleyl-imidazoline is preferred. The long chain amine usually contains 8-24 carbons and preferably is an aliphatic primary amine, which is especially saturated or mono ethylenically unsaturated; an example is dodecylamine. Mixtures of any of the above corrosion inhibitors with each other may be used, eg a quaternary ammonium salt and a long chain aliphatic hydrocarbyl-N-heterocyclic compound (where each is preferably as described above) , or mixtures with a tertiary aliphatic amine.

There may also be present with the Additive (i) at least one Additive (ix) which is at least one water soluble polymer of a polar ethylenically unsaturated compound different from Additive (i) and/or at least one Additive (x) which is a hydrophilic colloid. The Additive (ix) is usually water soluble to at least 10 g/1 at 20^BC and advantageously has a molecular weight of 1000-1500,000, e.g. 5000- 1,000,000, preferably 200,000-1,000,000 and especially 400,000- 900,000. The ethylenically unsaturated compound is preferably a vinyl or methyl vinyl group, and the polar group may be an alcohol, carboxylic acid, sulphonic acid or N-heterocyclic group, especially pyrrolidone. Preferred polar compounds are thus vinyl sulphonic acid, acrylic and methacrylic acids and N-vinyl pyrrolidone and "vinyl alcohol". The polymers may be copolymers, but are preferably homopolymers of these polar compounds, especially polyvinyl alcohol (e.g. hydrolysed polyvinyl acetate), polyacrylates and polyvinyl pyrrolidone (PVP). The amount of said polymer is usually 10-1000%, such as 50-300% or 90-250% based on the weight of the total of Additive(s) (i) - (v) and (vii) - (viii).

The hydrophilic colloid Additive (x) is an organic solid which is soluble in boiling water, e.g. to at least 10 g/1 or dispersible in boiling water and may be soluble (at least 10 g/1) or dispersible in water at 20^aC. It usually absorbs water strongly, e.g. to at least three times such as 3-15 times its weight of water at 20^BC, and swells in water. It can form a colloidal solution or dispersion in water and may have an average molecular weight of at least 10,000, e.g. 100,000-10,000,000. It may be a polysaccharide, e.g. with at least 4 carbohydrate units, especially one with at least some galactose units, e.g. 20-60% of such units, and may contain carboxylic acid residues, so that an aqueous solution or dispersion thereof can have an acidic reaction. The polysaccharide may be a natural gum, e.g. guar, agar, arabic, locust bean, karaya, carob or tragacanth gum, or a cellulosic material, such as starch, which may be unmodified or modified as an alkyl ether, e.g. methyl or ethyl cellulose or hydroxyalkyl ether, e.g. hydroxyethyl cellulose or carboxy alkylated starch, e.g. carboxy methyl cellulose (CMC). The polysaccharide may also be a synthetic, e.g. biosynthetic gum, the result of a microbiological process, e.g. fermentation; xanthan gum, which can be made by fermentation of dextrose with Xanthomonas campestris cultures, which is preferred, especially water soluble versions of xanthan gum. The colloid may also be proteinaceous , in particular gelatin or carrageenan (a seaweed extract), e.g. x- carrageenan. The colloid may also be a polyuronic acid or salt thereof, e.g. sodium or ammonium salt or ester thereof, such as a hydroxy alkyl ester (e.g. of propylene glycol), especially with beta- D-mannuronic acid residues; alginic acid and especially sodium alginate is preferred. The amount of Additive (x) may be 10 - 1000%, eg 50 - 300% or 90 - 250% by weight based on the total weight of Additives (i) - (v) and (vii) - (viii).

Mixtures of Additive (i) with more than one of thesame type (ii), (iii), (iv), (v) , (vi) , (vii), (viii), (ix) or (x) may be used, but also mixtures with ones of different types (ii) , (iii) , (iv) , (v) , (vi) , (vii), (viii), (ix) and/or (x) , especially mixtures of (i) with (viii) and at least one of (iii), (iv) , (v) , (vi) , (ix) and (x) . The mixtures may contain at least 1% by weight, but preferably at least 2% and not more than 90% of each Additive, especially in the form of two component mixtures with 25-75; 75-25 ratios of the two components. Such mixtures form another aspect of the invention, so there is also provided a blend or formulation which comprises a mixture of at least two Additives (i), (ii) , (iii), (iv) , (v) , (vi) , (vii), (viii), (ix) and (x) so long as it comprises at least one Additive (i) eg in amount of at least 1% by weight. Such blends are primarily for use as gas hydrate inhibitors. Additives (ii-viii) for use as hydrate inhibitors are preferably water soluble, e.g. to at least 10 g/1 in water at 20^aC. They may be used undiluted, but preferably are in solution such as aqueous solution, for example, as a solution in brine, or preferably an alcohol, for example, a water miscible one such as methanol or ethanol. Preferably are used Additives i-vii, an aqueous solution of which has a pH 1.5-12, e.g. 4-9, either naturally or after adjustment of the pH. Additives (viii) are preferably used in alcoholic solution while Additive (iii) is preferably used in acetic acid solution. Each Additive is suitably injected at concentrations in the range 10 to 20,000 ppm, e.g. 30 to 10,000 ppm, especially 50-1200 ppm based on the total water volume in the medium, in which hydrate formation is to be inhibited, in particular at concentrations in the range 100-700 ppm for Additive (i) - (v) and vii, 30-200 ppm for Additive (viii) and 300-5000 ppm for Additive (vi) . The amount of methanol, ethanol, or mono, di or tri ethylene glycol added relative to the total water volume in the medium is usually less than 10%, e.g. less than 5% or 2%, but especially less than 10,000 ppm, eg 1000 - 8000 ppm. The inhibitors may be injected at normal ambient conditions of temperature and pressure.

Formulations comprising Additive (i) and preferably at least one of Additive (ii) - (x) may be used in total amount of 50-10,000 ppm, especially 150-2000 ppm, e.g. 500-1500 ppm relative to the total water in the medium in which hydrates may form (including any water added in the formulation) . The Formulations may also comprise at least one Additive (iii)-(x). They may comprise the corrosion inhibitor (Additive (viii)) and a polymer additive which is the Additive (ix) eg PVP or a polymeric Additive (iii) eg chitosan as well as the Additive (i) , and optionally Additive (vi) which are then preferably in the weight ratio 5-200:100-2000:100-2000:0-5000, especially 10-150:400-800:400-800:1000-5000 or as percentages of the total formulation weight of corrosion inhibitor, polymer Additive (ix) or (iii), and Additive (i) of 1-20%, 40-80% and 10-60% respectively, especially 1-10%, 40-70% and 20-50% respectively. In the Formulation Additive (viii) is usually present in an amount of 0.1-50% (by weight of the total of Additive(s) (i) , e.g. 1-25%, especially 5-20%.

The Formulation may also contain another hydrate inhibitor and/or a water dispersant or surfactant, in particular an anionic one such as sodium dodecyl sulphonate or stearic acid and in amount of 1- 10% of the Formulation weight and/or a biocide, e.g. formaldehyde, e.g. in amount of 10-10,000 ppm and/or a metal complexant such as citric acid (e.g. in amount of 10-10,000 ppm) all amounts being in relation to the total weight of the Formulation. The Formulations may be used to retard or inhibit hydrate formation in the same manner as the individual Additives , as described above.

The inhibitor Additives and Formulations of the present invention are suitable for use in media containing water and gas, in particular in the petroleum, natural gas and gas industries. The gas may be a hydrocarbon normally gaseous at 25°C and 100 KPa pressure, such as an alkane of 1-4 carbon atoms eg methane, ethane, propane n or isobutane, or an alkane of 2-4 carbon atoms eg ethylene, propylene, n- or isobutene; the gas preferably comprises by weight at least 80% and especially at least 90% of methane with 0.1 - 10% eg 1- 5% C₂ hydrocarbon and 0.01 - 10% eg 0.05 - 5% C3 hydrocarbon. A natural gas, which may or may not have been purified or processed is preferred. The gas may also contain nitrogen eg in amount of 0.01-3% by weight and/or carbon dioxide eg in amount of 0.1 - 5% such as 0.5 - 2% by weight. The Additive (i) and its blends can be more effective than poly vinyl pyrrolidone in the inhibition of gas hydrate formation, especially when the gas comprises carbon dioxide eg. 1-5% by weight. In particular, they may be suitable for use during the transportation of fluids comprising gas and water eg from oil or gas wells. They may also be suitable for use in oil based drilling muds to inhibit hydrate formation during drilling operations.

In another aspect therefore the invention provides an oil based drilling mud, which comprises as hydrate inhibitor at least one Additive (i), as such or in a Formulation.

When used during the transportation of fluids, e.g. gases with water and optionally oil in conduits such as pipelines the inhibitors may be injected continuously or batchwise into the conduit upstream of conditions wherein hydrate formation may occur. Conditions under which gas hydrates may form as usually at greater than -5°C eg greater than 0°C such as 0 to 15°C eg 1 - 10°C and pressures eg of 0.1 - 30 MPa eg 1 - 15 MPa the temperature of onset of gas hydrate formation depending on the pressure, and the presence of, and concentration of, salt in the water. As the temperature decreases and the pressure increases and the concentration of salt decreases the greater is the likelihood for hydrate formation to happen in the absence of the Additives and Formulations of the invention. Thus the conditions of use of the Additive are ones such that in its absence a gas hydrate may form. In the absence of the Additive ice may also form in addition to the gas hydrate especially at temperatures of -5°C to 5°C. The pH of the water eg in the pipeline after addition of the Additives or Formulation is usually 3-9, especially 3.3-5 or 5-7.5. In drilling operations the inhibitors may be added to the drilling muds in the mud tank at the wellhead.

The invention is illustrated in the following Examples. Examples

To assess the efficiency of hydrate inhibitors suitable for use in the method of the present invention, tests were carried out using the following procedure:

The hydrate inhibitor test apparatus consisted of a simple 316 stainless steel pressure cell, with a usable internal volume of 1000cm³ with a thermostated cooling jacket, a sapphire window, an inlet and outlet and a platinum resistance thermometer. The cell contained water which was stirred by a magnetic pellet. Temperature and pressure were monitored and the results provided on a computer data logger; gas hydrates were also detected visibly using a time lapse video recording system. Before each test the cell was cleaned thoroughly by soaking successively in 10% aqueous hydrochloric acid for 1 hour, 10% aqueous sodium hydroxide solution for 1 hour and then double distilled water.

Into the cell was placed 200 cm³ of pre-chilled double distilled water with or without the chemical to be tested. A PTFE stirrer pellet was then placed in the cell and the pH of the solution measured with subsequent adjustment if desired by the addition of small but concentrated amounts of hydrochloric acid or sodium hydroxide. After sealing the cell the water was then stirred at 500 rpm and allowed to cool to the operational temperature of 4^sC. When this temperature was reached the stirrer was stopped and the video recorder started. Methane was then admitted to the cell until the pressure reached 70 bar (7 MPa) and the temperature, pressure and time were noted. The stirrer was restarted to run at 500 rpm and the time noted. Hydrates were observed to form in the vessel when the solution in the vessel turned opaque, coincident with which was a sharp temperature increase of about 0.2^aC and a gradual pressure reduction. The time from first contact of water and gas to formation of hydrate was read from the logger.

The experimental conditions are a very severe and accelerated test of gas hydrate formation and inhibition. The amounts of the Additive are expressed in ppm based on the volume of water. The inhibition time results given are an average of several results. Experiments A-F

In Experiments A/B and C/D the chemical tested was polyvinyl pyrrolidone (Molecular Weight 10,000 and 25000 respectively). In Experiments E and F the chemical tested was a "butylated polyvinyl pyrrolidone" sold under the Trade Mark "Antaron" P 904 by International Speciality Products of Wayne, N.J., USA, which is a water soluble copolymer powder with structural units derived from 90% N-vinyl pyrrolidone and 10% butylene and prepared by polymerization with t-butylperoxide; it is believed to be a graft copolymer of poly vinyl pyrrolidone (K30) and butylene, with the butylene structural units in the pyrrolidone ring and/or part of the hydrocarbon backbone and/or pendant thereon. The chemical has a weight average molecular weight of about 16000. The results were as follows.

TABLE 1

Example Additive Concentration pH Inhibition Time ppm approx (mins)

Blank None - 5-9

A PVP (10,000) 1000 4.5 17

B PVP (10,000) 400 - 10

C PVP (25,000) 1000 - 8, 20

D PVP (25,000) 400 - 5, 8, 20

E Antaron 904 1000 5.8 32

F Antaron 904 500 7.1 31.1 Examples 1 and 2

The process of Experiment F was repeated with (for Example 1) a blend of the Antaron P 904 polymer and Xanthan gum (water soluble powder from Sigma Chemical) each in amounts of 500 ppm (relative to the water) and (for Example 2) a blend of the Antaron P 904 and a corrosion inhibitor (sold under the Trade Mark "Champion" RU189 by Champion Technologies Inc, Texas, USA, which is believed to be a 1:1 mixture of quaternary ammonium salt and an aliphatic imidazoline) in amount of 500 and 65 ppm respectively.

The results were as follows:

Example Approx pH Inhibition Time mins

1 103

2 6.4 65

Examples 3-7

The process of Experiment F was repeated with 500 ppm Antaron P 904 in each Example mixed with other Additives, in amounts specified below. The additives were chitosan of Molecular Weight about 70,000 from Fluka dissolved in twice its weight of acetic acid, xanthan gum (as in Example 1), guar gum, and a corrosion inhibitor which was Benzalkonium chloride (benzyl alkyl dimethylammonium chlorides from Fluka believed to have 8-18 C alkyls), "Champion" RU189 or an alkyl benzyl dimethylammonium chloride sold by Hoechst under the Trade Mark DODIGEN 5462 or a corrosion inhibitor blend sold by Nalco as "Nal 1272", believed to contain tertiary amine, and/or quaternary ammonium salt and/or aliphatic imidazoline.

The results were as given in Table 2, in which the amounts of Additive shown are in addition to the 500 ppm of "Antaron" P 904.

Example 9

In a modification of Example 3 the inhibition time was tested when the water contained 950 ppm Antaron P 904, 950 ppm chitosan (with 3800 ppm acetic acid) 95 ppm Benzalkonium chloride and also the mono butyl ether of ethylene glycol (BGE) (3600 ppm) and methanol (6000 ppm), all the ppm amounts being relative to the water. A blend was made of chitosan (2.5g) acetic acid lOg, "Antaron" P 904 2.5g, the Benzalkonium chloride (0.25g) BGE (18g) methanol (30g) and water (24.4g) to give a concentrate, which was diluted to 100 ml with double distilled water. For use in the test, 8g of the above concentrate was diluted to 200 ml with the double distilled water.

In this Example the gas was not the methane of Example 3, but rather a synthetic gas mixture (simulating a particular type of natural gases) comprising by weight 95.3% methane, 3% ethane, 0.5% propane, 1% carbon dioxide and 0.1% each of isobutane and n-butane. The pH of the medium was 4.0. The test was performed as described in Example 3 apart from use of 5°C as the operational temperature. The average gas hydrate inhibition time over 6 runs was 138 mins For comparison the blank result with just the water and none of the extra ingredients was 6 mins under these conditions with this gas.

Examples 10 and 11 The process of Experiments E and F with 1000 and 500 ppm

"Antaron" 904 was repeated with the gas of Example 9 and under the conditions described in Example 9. The average gas hydrate inhibition times were 36 and 21 mins respectively. Example 12 and 13 The process of Example 9 was repeated with 20,000 ppm methanol and without the monobutyl ether of ethylene glycol, and (in Example 12) with the amounts of the other chemicals as in Example 9, and (in Example 13) with half those amounts eg 475 ppm "Antaron" P 904. The pH was about 3.5. The gas hydrate inhibition times were 320 min and 150 min respectively. For comparison the corresponding inhibition time with just the water and 100,000 p'pm methanol was 20 min. Examples 14 and 15

The process of Example 3 was repeated with the 500 ppm of "Antaron" P 904 product replaced by 500 ppm of "Antaron" V216 product from International Speciality Products, Wayne, N.J., USA, which is the corresponding "alkylated polyvinyl pyrrolidone" with structural units derived from hexadec-1-ene rather than butylene; "Antaron" V216 product is less soluble in water than "Antaron" P 904, but is water dispersible. The other additives were as listed below in which "oleyl imidazoline" denotes N-2-aminoethyl-2-oleyl-imidazoline acetate sold by BP Chemicals Limited as a corrosion inhibitor "IC

Claims

Claims :

1. A method for inhibiting or retarding hydrate formation, which method comprises adding an additive (hereinafter called the Additive) which is (i) an aliphatic (N-heterocyclic carbonyl) polymer, hereafter called the Polymer, the Additive being added in an amount effective to inhibit or retard hydrate formation, to a medium susceptible to hydrate formation, said medium being a gas comprising 0.5-5% by weight of carbon dioxide.

2. A method for inhibiting or retarding hydrate formation, which method comprises adding an additive (hereinafter called the Additive) which is (i) an aliphatic (N-heterocyclic carbonyl) polymer, hereafter called the Polymer, and at least one of (ii) a cyclic compound with at least -(C=0)-0- group in a ring, (iii) an amino carbohydrate, (iv) an amino acid or derivative thereof, (v) an amino alcohol or derivative thereof, (vi) a hydroxy ether or derivative thereof (vii) a hydroxy acid or a derivative thereof, (viii) a quaternary ammonium salt corrosion inhibitor, (ix) a water soluble polymer of a polar ethylenically unsaturated compound different from Additive (i) and (x) a hydrophilic colloid, the Additive being added in an amount effective to inhibit or retard hydrate formation, to a medium susceptible to hydrate formation.

3. A method according to claim 2 wherein said medium is a gas comprising 0.5-5% carbon dioxide.

4. A method according to any one of the preceding claims, wherein said Polymer (i) is an alkylated polyvinyl N-pyrrolidone polymer.

5. A method according to claim 4 wherein said polymer (i) is a butylated or hexadecylated polyvinyl pyrrolidone polymer.

6. A method according to any one of the preceding claims, which comprises adding said Polymer (i) , and at least one of (x) a hydrophilic colloid and (iii) an amino carbohydrate.

7. A method according to claim 6 wherein said colloid (x) is a xanthan or guar gum.

8. A method according to claims 6 or 7 wherein said amino carbohydrate (iii) is chitosan.

9. A method according to any one of the preceding claims which also comprises adding a corrosion inhibitor to said medium.

10. A Formulation for use as a gas hydrate inhibitor which comprises blend of (i) and at least one of Additives (ii)-(x) said Additives being defined in anyone of claims 2 and 4-9.

11. Method of transporting through a pipeline a fluid comprising water and a gas susceptible to form hydrates, which comprises treating said fluid with Additive (i) and at least one Additive (ii)- (x) as defined in any one of claims 1, 2 and 4-9 or with a formulation as defined in claim 10 upstream of a locus in said pipeline wherein gas hydrates may be formed in the absence of said Additives.

12. Use of Additive (i) and at least one of Additives (ii)-(x) as defined in any one of claims 1, 2 and 4-9 or a blend as defined in claim 8 for the inhibition or retardation of the formation of gas hydrates.

13. An oil based drilling mud, which comprises a hydrate inhibitor which is Additive (i) and at least one Additive as defined in any one of claims 1, 2 and 4-9 or a formulation as defined in claim 10.